Functional membrane properties of chick heart change dramatically between the time the organ begins beating (36 hrs) and the period when it has established its definitive 4-chambered structure (7 days). We will analyze two specific membrane systems that differentiate during this period: the tetrodotoxin-sensitive fast sodium channel, and an insulin-sensitive amino acid transport mechanism. The development of both systems will be investigated in embryonic chick heart cells under a variety of different culture conditions, for which a body of background information already exists. Hearts will be dissociated into their component cells and cultured as spontaneously beating spheroidal aggregates. The appearance of TTX-sensitive Na-specific channels will be assessed by current-voltage curves and voltage-clamp analysis of preparations taken from different aged embryos. Similar analyses will be performed on cells exposed to current-blocking agents, inhibitors or protein synthesis, and proteolytic enzymes. Similar heart-cell preparations will be used to investigate the developmental control mechanisms underlying an amino acid transport system, its differentiation and the molecular mechanisms that regulate the appearance of both insulin-responsive and non-responsive components. The kinetics of uptake and efflux of labeled amino acid analogs will be studied in cells from different aged hearts, subjected to variations in substrate concentration, ion content, pH, temperature, competitive and non-competitive inhibitors of transcription and translation.